In the manufacture of fiber reinforced resin products, sheet molding compounds are frequently used. Sheet molding compounds offer an appealing solution for the production of Class A surface parts compared to steel in terms of cost, weight, styling flexibility and corrosion resistance.
Sheet molding compounds consist of a mixture of a thermosetting resin, a thermoplastic (typically dissolved in styrene) and catalyst, particulate filler and chopped reinforcement fibers, such as glass fibers. In most cases, the resin and chopped fibers are sandwiched between films of plastic material to form a laminated sheet that is wound in rolled form or festooned for storage. The laminated sheet is stored under conditions that will not result in final curing of the resin, but will allow the paste to thicken from typically a 10,000 to 40,000 centipoise (milliPascal seconds-mPa·s) range to a desired molding viscosity range, typically between 30,000,000 and 50,000,000 centipoise (mPa·s). At the time of use, the protective carrier film is removed and the laminated sheet is cut into blanks, or plies, of a desired shape and size. The plies are then molded to form a cured composite part. In most applications, multiple plies of the laminated sheets are used in the composite structure and typically comprise between 25 and 50% of the die/tool's surface area. When the laminated sheets are molded, the resin and glass flow within the mold under heat and pressure to cover the entire surface of the mold. Sheet molding compounds are used in a variety of applications that require aesthetic appeal, corrosion resistance, lighter weight dimensional control and high strength.
One deficiency with currently available sheet molding compounds is that the charge may not form a Class A type surface part when molded and cured. This is due to the fact that sometimes the chopped fibers move to the surface of the sheet molding compound to form surface imperfections. Further, the sheet molding compound sometimes does not flow well in the mold, especially if it is loaded incorrectly, and this creates surface imperfections such as surface pores. Thus, some SMC composite parts may be scrapped, or require sanding and repair, or must otherwise be reworked to be used in applications requiring a desired surface appearance.
Yet another problem with surface characteristics occurs when these composite parts formed from the sheet molding plies are painted. Paint pops may be caused by the release of volatile liquids (such as water, styrene or di-vinyl benzene monomer) from the sheet molding paste or by the release of moisture or solvents contained in the resin or within/around fiber bundles during the curing process. These are quite common, typically affecting 5–10% or more of painted SMC composite parts, leading to substantial cost in terms of rework and waste.
In addition, to allow a long and uniform flow that will produce a wave-free surface, the fibers used in sheet molding compounds are typically provided by the glass manufacturer as bundles or “splits” of multiple filaments. The act of impregnating the bed of chopped fibers between two layers of sheet molding compound paste often leaves air trapped within the composite sheet, often beside or between the bundles, where small differences in surface tension adversely affects the wetting of the bundles or splits. Unfortunately, this bundling may also include entrapped air which, when released during the flow, produces tiny bubbles which travel slowly under a pressure gradient. To evacuate these bubbles, it is useful to have the molding compound flow to fill out the tool to allow the action of the pressure gradients to move those air bubbles towards the edge of the flow front and thus towards the edge of the part. Such large flow typically calls for loading the tool by a charge representing 50% or less of the area of the part. Others have shown that molding under vacuum may aid in the removal of extraneous gases.
In addition, the flow of the various stacked sheets in the charge is not homogeneous. The sheets in the center of the stack flow more slowly than those in contact with the tool (mold). This does not allow the use of special treatment to the top layer to solve the problem of paint popping.
Another problem with currently available sheet molding compounds is that they are difficult to paint along with steel parts in a conventional assembly line painting system. Typically, composite parts require the use of a conductive primer applied prior to the application of an ornamental surface paint. To improve electrostatic sprayability, conductive materials have been introduced to sheet molding compounds. However, typical sheet molding compounds require large amounts of conductive materials to be introduced in order to improve surface conductivity enough to be effectively electrostatically sprayable. This increases raw material costs and can decrease surface quality associated with increased fiber loading.
One potential way to produce sheet molding compounds has been to locally sandwich a small piece of wet process textile mat, instead of chopped fibers, in a localized area between layers of sheet molding compound paste and molding the resultant laminate into a composite part. However, the fiber contained within a conventional wet process textile veil mat does not flow well under pressure, and is not intended to do so. Thus, the composite parts formed by this process have similar poor surface characteristics as composite parts formed with chopped fibers.
U.S. Pat. No. 4,302,499 illustrates a method of making a moldable composite, including the steps of providing a fabric material on the outside of an SMC sheet, as shown in FIGS. 3 and 5 of the '499 patent, to provide corrosion resistance and avoid fiber read-through. In the '499 patent, the resin flows through the veil during molding to form a resin-rich layer on the outside of the veil, but no resin is provided on the outside of the veil. Additionally, the '499 patent indicates the properties of the veil are critical, and describes these properties to include that the veil must be permeable so the resin flows through the veil during the molding process and that the veil must posses a grab break strength of at least ten pounds per inch in both longitudinal and transverse directions so as to avoid tearing during compression molding. Thus, the '499 patent includes a system as shown in FIG. 2, where the veil is draped over the entire tool surface, and the veil is strong enough to remain intact without tearing, while the veil does not apparently flow with the resin during molding. The second embodiment shown in FIGS. 4 and 5 is further described in Example 1, and uses a synthetic fabric (Nexus), capable of substantial elongation, but which is molded at relatively low temperature (275 F), and which provides the fabric on the outer surface of the resin, and where the resin flows through the veil during molding. The '499 patent does not teach a method for making a Class A surface part.
A further deficiency in the current technology is that sheet weight is limited due to the currently available manufacturing techniques. Typically resin, or sheet molding paste, is applied to two carrier film layers using a doctor blade, and a layer of chopped glass or glass mat is sandwiched between the two resin-coated layers to form a composite sheet. One such sheet molding compounding line is shown in U.S. Pat. No. 6,251,974 to Rossi et al., which is herein incorporated by reference. Another such sheet molding compounding line is shown in U.S. Pat. Nos. 6,103,032 and 6,119,750 to Greve, assigned to the Budd Company, which are herein incorporated by reference. The use of doctor blades, usually an upside down weir blade, to apply the sheet molding paste allows only two layers of sheet molding paste to be used, since the prior art teaches that a carrier film passes below the doctor blade to form each layer of paste, and this carrier film must be at the outside of the sheet to be removed and disposed prior to molding. Accordingly, because there is only two resin layers, the need to penetrate the sheet molding paste through the fiber package while maintaining proper paste/fiber distribution limits the thickness achievable. Thus, in the prior art, several layers of the composite sheet are required to be placed in a tool to provide a charge having the desired amount of material for the composite part.
Yet another deficiency in the current technology is that sheet weight is inconsistent. This is attributed to the viscosity changes in the sheet molding paste attributed to thickening and the use of doctor blade or other paste dispensing devices as metering tools whose throughput rates are dependent upon the viscosity of the metered media. Consequently, in order to manufacture a part having a constant weight and volume, the charge size must be adjusted from part to part, which leads to inconsistencies in flow and performance of composite parts.
It is therefore highly desirable to improve the characteristics of sheet molding compound. This would allow sheet molding compound parts to be used in a wider variety of composite applications wherein surface quality as well as scrap and rework is a concern.
It is thus also highly desirable to improve the electrostatic sprayability of composite parts made of sheet molding compounds.